U.S. patent number 7,383,739 [Application Number 11/409,876] was granted by the patent office on 2008-06-10 for magnetoinductive flowmeter having an electrode with an electroconductive diamond coating.
This patent grant is currently assigned to Krohne Messtechnik GmbH Co. KG. Invention is credited to Friedrich Hofmann.
United States Patent |
7,383,739 |
Hofmann |
June 10, 2008 |
**Please see images for:
( Certificate of Correction ) ** |
Magnetoinductive flowmeter having an electrode with an
electroconductive diamond coating
Abstract
A magnetoinductive flowmeter includes a measuring tube
containing an electrode that is provided with an electroconductive
diamond coating, as a result of which the galvanic contact between
the electrode and the medium that flows through the measuring tube
is indirect, i.e. via the electroconductive diamond coating. This
produces an electrode that offers mechanical strength and is
conducive to interference-free measuring signals. A method for
producing the flowmeter is also disclosed.
Inventors: |
Hofmann; Friedrich (Straelen,
DE) |
Assignee: |
Krohne Messtechnik GmbH Co. KG
(Duisburg, DE)
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Family
ID: |
37085031 |
Appl.
No.: |
11/409,876 |
Filed: |
April 24, 2006 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20060236780 A1 |
Oct 26, 2006 |
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Foreign Application Priority Data
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Apr 25, 2005 [DE] |
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10 2005 019 418 |
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Current U.S.
Class: |
73/861.12 |
Current CPC
Class: |
G01F
1/584 (20130101) |
Current International
Class: |
G01F
1/58 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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4105311 |
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Aug 1992 |
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DE |
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WO 9855837 |
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Dec 1998 |
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WO |
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Primary Examiner: Patel; Harshad
Attorney, Agent or Firm: Cesari and McKenna, LLP
Claims
The invention claimed is:
1. A magnetoinductive flowmeter, with a measuring tube that
conducts a flowing medium and, situated in said measuring tube, an
electrode that establishes galvanic contact with the flowing
medium, wherein said electrode is provided with an
electroconductive diamond coating.
2. The magnetoinductive flowmeter as in claim 1, wherein the
electroconductive diamond coating completely covers the surface
area of the electrode that comes in contact with the flowing
medium.
3. The magnetoinductive flowmeter as in claim 1 or 2, wherein the
diamond coating is composed of a doped polycrystalline diamond
layer.
4. The magnetoinductive flowmeter as in claim 1 or 2, wherein the
diamond coating is composed of a doped ADLC layer.
5. The magnetoinductive flowmeter as in claim 1 or 2, wherein the
diamond coating includes a boron dopant.
6. The magnetoinductive flowmeter as in claim 1 or 2, wherein the
diamond coating is applied by means of a CVD process.
7. The magnetoinductive flowmeter as in claim 1 or 2, wherein the
area of the electrode to which the diamond coating is applied
consists of niobium, tantalum or tungsten.
8. The magnetoinductive flowmeter as in claim 1 or 2, wherein the
diamond coating exhibits a resistivity in the range from 10.sup.5
to 10 .OMEGA.m.
9. A method for producing a magnetoinductive flowmeter comprising
the steps of providing a measuring tube that conducts a flowing
medium, positioning an electrode in said tube so as to come in
galvanic contact with the flowing medium, and providing said
electrode with an electroconductive diamond coating.
10. The method for producing a magnetoinductive flowmeter as in
claim 9, wherein the providing step includes completely covering
with the electroconductive diamond coating the surface area of the
electrode that comes in contact with the flowing medium.
11. The method for producing a magnetoinductive flowmeter as in
claim 9 or 10, including the step of constituting the diamond
coating of a doped polycrystalline diamond layer.
12. The method for producing a magnetoinductive flowmeter as in
claim 9 or 10, including the step of constituting the diamond
coating of a doped ADLC layer.
13. The method for producing a magnetoinductive flowmeter as in
claim 9 or 10, including the step of doping the diamond coating
with boron obtained in particular by vaporizing boron metal or a
liquid source of boron such as trialkyl borate, or in the form of a
gas such as diborane.
14. The method for producing a magnetoinductive flowmeter as in
claim 9 or 10, including the step of applying the diamond coating
by means of a CVD process.
15. The method for producing a magnetoinductive flowmeter as in
claim 9 or 10, including the step of using niobium, tantalum or
tungsten for the area of the electrode to which the diamond coating
is applied.
Description
This invention relates to a magnetoinductive flowmeter
incorporating a measuring tube through which passes a flowing
medium, as well as an electrode that is provided in the measuring
tube and is in galvanic contact with the flowing medium. The
invention further relates to a method for producing a
magnetoinductive flowmeter, whereby an electrode is mounted in a
measuring tube, serving to conduct a flowing medium, in such
fashion that the electrode is in galvanic contact with the flowing
medium.
BACKGROUND OF THE INVENTION
Magnetoinductive flowmeters are well-known from prior art,
described, for example, in "K. B. Bonfig, Technische
Durchflussmessung (industrial flow measurements), 3.sup.rd edition,
Vulkan-Verlag, Essen, 2002, pp. 123-167". The underlying concept of
a magnetoinductive flow-measuring device goes all the way back to
Faraday who in 1832 proposed using the principle of electrodynamic
induction for flow-rate measurements.
According to Faraday's law of induction, a flowing medium that
contains charge carriers and travels through a magnetic field will
produce an electric field intensity perpendicular to the direction
of flow and perpendicular to the magnetic field. A magnetoinductive
flowmeter utilizes Faraday's law of induction in that by means of a
magnetic-field system containing at least one magnet with typically
two field coils a magnetic field is generated and positioned over
the cross-sectional area of the measuring tube, which magnetic
field includes a magnetic-field component that extends in a
direction perpendicular to the direction of flow. Within the
magnetic field, each volume element of the flowing medium,
containing a certain number of charge carriers, contributes the
field intensity created in that volume element to the measuring
voltage that can be collected via the electrodes.
One salient characteristic of magnetoinductive flowmeters is the
proportionality between the measured voltage and the velocity of
the flowing medium through the cross section of the measuring tube,
i.e. between the measured voltage and the volume flow. Apart from
the electrodes serving to collect the voltage being measured,
additional electrodes may be provided, such as zero-flow detection
electrodes as well as grounding electrodes especially also in the
form of grounding sleeves.
Metal electrodes that are in direct contact with the flowing medium
form an electrochemical boundary layer, capable of producing
electrochemical direct-current voltages whose order of magnitude
may be several 100 mV. These electrochemical direct-current
voltages can change quite rapidly, for instance as a function of
variations in the local flow rate of the medium at the electrodes
due to turbulences, of operating-pressure fluctuations, of the pH
value of the medium, of the composition of the medium especially
while chemical reactions are still going on in the medium, of solid
particles carried by or particles suspended in the medium and
interfering with the boundary layer on the electrodes, or of solid
particles in contact with or indeed impinging on the electrodes.
All these factors lead to statistical fluctuations of the
electrochemical direct current at the electrodes with amplitudes
ranging from a few .mu.V to several 10 mV. These statistical
voltage fluctuations are also referred to as random noise.
This random-noise voltage is superimposed on the flow-proportional
signal voltage whose signal intensity is typically 0.1 to 1
mV/(m/s), i.e. 0.1 to 1 mV for a flow velocity of one meter per
second. It follows that the amplitude of the random-noise voltage
may be of the same magnitude as the flow-proportional signal
voltage or even well above that. The result may be substantial
noise interference with consequently irregular, strongly
fluctuating flow-rate readings of the magnetoinductive
flowmeter.
That problem has been addressed in the prior art by applying on the
surface areas of the electrodes that come in contact with the
medium a porous coating consisting of a nonconducting, electrically
inert material, for instance a porous ceramic layer. The
electrically conductive medium penetrates into these pores,
creating a voltaic connection between the medium and the metal of
the electrodes and their associated transducer. This has a number
of beneficial effects:
The medium that makes contact with the metal of the electrodes is
replaced very slowly. This significantly minimizes sudden,
measurement-disrupting spikes of the direct-current voltage at the
electrodes as a result of an electrochemically heterogeneous
medium. Moreover, solid particles carried by the medium are no
longer able to penetrate into the electrochemical boundary layer
that is now largely protected by the ceramic coating. Solid
particles carried by the medium therefore cause significantly less
interference than would be the case without that porous ceramic
layer.
To be sure, this type of porous ceramic layer has drawbacks of its
own. For example, abrasion or physical impact can damage the
ceramic layer whenever the medium carries along hard solids. Then,
too, the chemical resistance of the ceramic material used is not
always adequate. The same is true for the thermal-shock resistance
of these ceramic materials. Finally, electrically insulating oils
or fats carried by the medium can penetrate into the pores of the
ceramic, interrupting the galvanic connection between the medium
and the metal electrode.
SUMMARY OF THE INVENTION
It is therefore the objective of this invention to introduce a
magnetoinductive flowmeter, and a method for producing a
magnetoinductive flowmeter, employing a type of electrode that
delivers a highly noise-free flow signal and offers a high level of
mechanical resistance.
For a magnetoinductive flowmeter as described at the outset, this
objective is achieved by providing the electrode with an
electrically conductive diamond coating.
Thus, according to the invention, any direct contact between the
medium and the electrode is avoided by coating the electrode with
an electroconductive diamond coating. Specifically, in a preferred
embodiment of the invention, the electroconductive diamond coating
completely covers the area of the electrode that is exposed to
contact with the flowing medium.
The result is an electrode for a magnetoinductive flowmeter that is
very hard and offers high abrasion resistance as well as a maximum
level of chemical stability. In addition, this electrode is
electrically and electrochemically inert, with the particular
absence of any piezoelectric effect, and there is no or only a
minimal build-up of electro-chemical tension.
These advantages notwithstanding it is still possible to produce an
electroconductive diamond coating that offers sufficiently high
electrical conductivity. Specifically, in a preferred embodiment of
the invention, the resistivity of the diamond coating is in the
range between 10.sup.-5 and 10 .OMEGA.m. That obviates the need for
through-pores in order to establish a galvanic connection from the
medium to the electrode.
For producing an electroconductive diamond coating there would be a
number of suitable materials. However, the diamond coating in a
preferred embodiment of the invention is composed of doped
polycrystalline diamonds. Alternatively, the diamond coating may be
a doped layer of amorphous diamond-like carbon materials (an ADLC
layer).
One preferred embodiment of the invention employs boron as the
dopant. Specifically, in a preferred implementation of the
invention, the diamond coating is applied by a CVD (chemical vapor
deposition) process.
The diamond-coated area of the electrode preferably consists of a
metal. Particular preference for the area to which the diamond
layer is applied is given to niobium, tantalum or tungsten. These
materials display an especially good affinity for a diamond coating
applied by a CVD process. Incidentally, it is not necessary for the
entire electrode to consist of the material on which the diamond
coating is deposited. Specifically, the electrode may be composed
of at least two different metals, one of them preferably being one
of the aforementioned metals to ensure the desired adhesion of the
diamond coating.
With reference to the method described at the outset, the stated
objective is achieved by coating the electrode with an electrically
conductive diamond layer.
Preferred enhancements of the method according to this invention
correspond in analogous fashion to the above-described preferred
embodiments of the inventive magnetoinductive flowmeter.
There are numerous different ways in which the magnetoinductive
flowmeter according to the invention and the inventive method for
producing a magnetoinductive flowmeter can be configured and
further enhanced. In this context, attention is invited to the
dependent claims and to the following detailed description of a
preferred embodiment of the invention with reference to the
attached drawing.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawing, the single FIGURE depicts schematically the
measuring tube of a magnetoinductive flowmeter, which measuring
tube contains an electrode that has been provided with an
electroconductive diamond coating.
DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT
As shown in the drawing FIGURE, the measuring tube 1 of a
magnetoinductive flowmeter contains an electrode 2 so positioned as
to make galvanic contact with the medium flowing through the
measuring tube. However, the electrode 2 proper is not in direct
contact with the flowing medium; instead, that contact is indirect,
via an electroconductive diamond coating 3 deposited on the surface
area of the electrode 2 facing the flowing medium. In the
embodiment example of the invention here described, the surface
area of the electrode 2 that would otherwise be in direct contact
with the flowing medium is completely covered with the
electroconductive diamond coating 3. Thus, instead of a direct
contact between the medium and the solid particles it carries and
the electrode 2, the latter in this case consisting of niobium,
there is only an indirect contact, established via the
electroconductive diamond coating 3.
In the preferred embodiment of the invention here described, the
electrically conductive diamond coating 3 consists of a
polycrystalline diamond layer doped with boron and deposited by a
CVD process. Boron doping allows for the resistivity parameter of
the electroconductive diamond layer 3 to be set within a range from
10.sup.-5 to 10 .OMEGA.m. This produces a coating with the typical
properties of diamond, these being chemical inertness, thermal
resistance up to at least 600.degree. C. and dimensional
stability.
The CVD process is typically applied at a pressure in the range
between 10 and several 100 mbar, employing electrically or
thermally activated hydrocarbon-containing vapors. For the coating,
the electrode 2 is maintained at a temperature between 500.degree.
C. and 1000.degree. C., permitting deposition rates of up to
several 100 .mu.m/h.
Doping with boron can be performed in several different ways, for
instance by vaporizing boron metal or a liquid source of boron such
as trialkyl borate. A gas such as diborane can also be used.
* * * * *